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  • To send an electrical signal, we need two wires.

  • Since two metal conductors are by definition a capacitor,

  • this means that each section of our transmission line has some capacitance.

  • And since a loop of wire is by definition an inductor,

  • each section of our line also has some inductance.

  • Suppose we close a switch applying a constant DC voltage across our two wires.

  • This capacitance and inductance is what prevents

  • the signal from travelling instantaneously.

  • Therefore, we can never eliminate this capacitance and inductance completely

  • because this would then violate Einstein's Theory of Relativity,

  • which states that information can never travel faster than the speed of light.

  • These capacitors and inductors shown

  • are not components that we deliberately add to a circuit,

  • but are an inherent part of all wires that carry electrical signals.

  • When electric circuits appear to behave

  • in unpredictable ways that seem to violate the laws of physics,

  • this is often due to the capacitance and the inductance of the wires.

  • In this instance, for example, the signal keeps bouncing back and forth

  • between the beginning and the end this transmission line.

  • With nothing connected to the end of the line,

  • the voltage at the end of the transmission line reaches

  • double the value of the DC input voltage from the battery,

  • and the signal is then reflected back.

  • Reflections can also occur at the connection between two transmission lines

  • which have different amounts of capacitance and inductance,

  • in which case only a portion of the signal's energy is transmitted to the next line,

  • and the rest of the energy is reflected back.

  • Suppose we connect a short circuit at the end of a transmission line.

  • What will happen when this signal reaches this short circuit?

  • As always, the speed with which a signal travels depends

  • on the capacitance and inductance of each section of the wire.

  • When the signal reaches the short circuit, the signal is reflected,

  • but with the voltage flipped upside down!

  • If the impedance at the end of the line is too low

  • it will behave like a short circuit,

  • and if the impedance is too high it will behave like an open circuit,

  • both of which cause reflections.

  • But, if we place just the right amount of impedance at the end of the line

  • for the amount of capacitance and inductance in each section of the line,

  • then the amount of reflection can be minimized.

  • Reducing reflections is desirable, as reflections can

  • significantly interfere with the proper operation of a circuit.

  • Much more information is available in the other videos on this channel,

  • and please subscribe for notifications when new videos are ready.

To send an electrical signal, we need two wires.

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B1 US signal capacitance circuit transmission line voltage

Transmission Lines - Signal Transmission and Reflection

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    Amy.Lin posted on 2021/01/06
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